1. Field of the Invention
The present invention relates to an inverter which drives a motor-driven compressor for use in a vehicle air conditioning apparatus. More particularly, this invention relates to a control method that effectively reduces the frequency with which the compressor motor is stopped due to an overload condition. Moreover, the present invention relates to an inverter configured to perform the control method of the present invention.
2. Description of Related Art
In FIG. 1, a known motor-driven compressor 1 and an inverter 2 for use in a vehicle air conditioning system are shown. Motor-driven compressor 1 comprises a compressor 11 and a three-phase, direct current (DC) motor 12, which drives compressor 11. Inverter 2 comprises a smoothing condenser 22, three pairs of switching elements 21a-21f (NPN-type transistors), a shunt resister 23, and a motor drive controller 24. Inverter 2 is supplied with DC electrical power from a DC battery 3. Motor drive controller 24 comprises a central processing unit (CPU) 26, an analog-to-digital (A/D) converter 25, a memory 27, a motor position detector 29, and a control signal generator 28. When a rotational frequency command signal 31 is input to CPU 26 from an external device (not shown), CPU 26 outputs a signal 32 to control signal generator 28 in accordance with a program stored in memory 27. Control signal generator 28 activates specific combinations of switching elements 21a-21f sequentially in accordance with a specific order.
When combinations of switching elements 21a-21f are activated sequentially, three-phase, DC current flows into the coils of motor 12, and motor 12 begins to rotate. As motor 12 rotates, a counter-electromotive force (back-emf) is generated on the terminals of motor 12. The back-emf is input to motor position detector 29, and then is translated into a signal 33 which provides an indication of a rotational position of a rotor of motor 12. When CPU 26 receives signal 33 from motor position detector 29 and calculates a rotational position of the rotor, CPU 26 generates a new signal 32 for controlling switching elements 21a-21f. Thus, CPU 26 may calculate rotational frequency of motor 12 based on signal 33 which is output from motor position detector 29. When a current flows into motor 12, it also flows through shunt resister 23, which is located on a return path of the current. This current Ip flowing through shunt resister 23 is called a phase current, and a potential difference develops across shunt resister 23. This potential difference is proportional to the phase current and is input to A/D converter 25. Thus, by monitoring the amplitude of current Ip, CPU 26 controls signal 32, which further controls switching elements 21a-21f, so that the current Ip does not exceed a certain level.
A known, main control program of inverter 2 may be stored in memory 27 of motor drive controller 24. This known control program operates as follows. As mentioned above, CPU 26 monitors the amplitude of the current Ip. When a load corresponding to an air conditioning parameter increases, e.g., when the ambient temperature rises, a load on compressor 11 and on motor 12 also increases. Generally, the current Ip, which flows through motor 12, is proportional to a rotational load on motor 12.
By monitoring the amplitude of the current Ip, CPU 26 may detect that a rotational load on motor 12 has increased. The amplitude of the current Ip, which may flow through motor 12 and inverter 2, is limited by (1) a rated current of motor 12; (2) a rated current of switching elements 21a-21f; (3) a rated current of conducting wires, which connect these devices; and (4) a rated current of the connectors between these devices. When an amplitude of current Ip approaches the rated current of motor 12 and inverter 2, CPU 26 then lowers the rotational frequency of motor 12. However, because the magnitude of the load on motor 12 varies, after a period of delay, in response to a change in the rotational frequency of motor 12, the motor load continues to increase for some period after the rotational frequency of motor 12 is lowered. As a result, the amplitude of the phase current Ip may continue to increase, and it may exceed a rated current of inverter 2 or motor 12.
When the amplitude of the phase current Ip exceeds a rated current of inverter 2 or motor 12, CPU 26 ceases to activate control signal generator 28 and, thus, switching elements 21a-21f. This action by CPU 26 stops drive motor 12. If motor 12 is stopped abruptly during the operation of the vehicle air conditioning system, warm air may be discharged from a port of the air conditioning system. In addition, motor 12 may be restarted after a predetermined period of time elapses from the time at which motor 12 was first stopped. Thus, according to known methods for controlling an inverter, motor stoppage may occur more frequently whenever an amplitude of the current Ip approaches a limit value, i.e., a rated value of inverter 2 or motor 12.
A need has arisen to provide an improved control method for an inverter that drives a motor-driven compressor. A further need has arisen for a control method that reduces the frequency with which a motor is stopped upon the approach of an overload condition. Moreover, a need has arisen to provide an inverter configured to perform this control method. According to the control method of the present invention, at least five set points related to the amplitude of the phase current may be preset. The first set point among the five set points is one of (1) the rated current of the switching elements, (2) the rated current of the motor, or (3) the rated current of the conducting wires and connectors which connect these devices. According to the method of the present invention, when the amplitude of the phase current exceeds this first or greatest set point, activation of the motor is stopped. When the amplitude of the phase current falls below the lowest of the five set points, the rotational frequency of the motor is accelerated by a predetermined degree of acceleration.
The remaining three intermediate set points are located between the first set point and the lowest set point. The intermediate set points define subintervals between the first set point and the lowest set point. When the amplitude of the phase current enters one of the subintervals defined by these three intermediate set points, the motor is accelerated by various negative degrees of accelerations (i.e., the motor is decelerated) predetermined and defined for each of the subintervals. Because the control method of the present invention steadily decreases rotational velocity of the motor by a certain deceleration before the amplitude of the phase current exceeds the first or largest set point, the method of the present invention effectively reduces the occurrence of motor stoppage as an overload condition approaches.
In an embodiment of this invention, a control device of an inverter includes a plurality of switching elements for driving a three-phase DC motor in response to a rotational frequency, command signal received from an external device. The rotational velocity and the rotational acceleration of the motor are determined from signals which are in response to a back-emf received from the DC motor as it rotates. The amplitude of the phase current of the motor is determined, as well. When the amplitude of the phase current exceeds a first set point, activation of the motor is stopped. When the amplitude of the phase current is less than the first set point but greater than a second set point, which is less than the first set point, the rotational velocity of the motor is decelerated at a predetermined rate. When the amplitude of the phase current is less than a third set point, which is smaller than the second set point, the rotational velocity of the motor is accelerated at a predetermined rate.
In another embodiment of this invention, an inverter for performing the control method of the present invention is provided. The inverter includes a plurality of switching elements for driving a three-phase DC motor based on a rotational frequency command signal received from an external device. A calculator is provided for calculating rotational velocity and rotational acceleration of the motor from signals, which are based on a back-emf received from the motor as it rotates. A detector is provided for determining an amplitude of the phase current of the motor. Moreover, a control program having an overload suppression routine is provided. The overload suppression routine operates as follows. When the amplitude of the phase current exceeds a first set point, the routine stops activation of the motor. When the amplitude of the phase current is less than the first set point, but greater than a second set point, which is smaller than the first set point, the routine decelerates the rotational velocity of the motor at a predetermined rate. When the amplitude of the phase current is less than a third set point, which is smaller than the second set point, the routine accelerates the rotational velocity of the motor at a predetermined rate.
In a further embodiment of this invention, a method of controlling an inverter-driven compressor motor is provided. The method of the present invention calculates a rotational velocity and a rotational acceleration of the motor, in response to back-emf signals of the motor. Moreover, the method detects an amplitude of a phase current of the motor. When the amplitude of the phase current exceeds a first set point (e.g., first set point I1), the control method stops activation of the motor. When the amplitude of the phase current is less than the first set point (e.g., first set point I1), but greater than a second set point (e.g., second set point xcex1I2), which is less than the first set point (e.g., first set point I1), the control method decelerates the rotational velocity of the motor at a predetermined rate. Moreover, when the amplitude of the phase current is less than a third set point xcex1I3, which is less than the second set point xcex1I2, the control method accelerates the rotational velocity of the motor at a predetermined rate.
Other objects, features, and advantages of embodiments of this invention will be apparent to, and understood by, persons of ordinary skill in the art from the following description of preferred embodiments with reference to the accompanying drawings.